Nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) which are the three toxic components of engine exhaust gas, are efficiently purified by installing a three-way catalyst in the exhaust passage.
However, if there is a leak in the exhaust passage leading from the engine to the catalyst, the toxic components are discharged into the atmosphere without being purified.
Japanese Patent No. 3075079 published by the Japanese Patent Office on Jun. 9, 2002, discloses a technique for diagnosing the presence of a leak in this exhaust passage. This prior art technique is applied to an air-fuel ratio feedback control device which feedback controls the air-fuel ratio of the gaseous mixture burnt by the engine based on the output of an oxygen sensor which detects the oxygen concentration in the exhaust gas. The prior art technique learns an air-fuel ratio feedback correction coefficient respectively for a low load region and a high load region and determines the presence or absence of an exhaust gas leak by comparing the two learned values.